In SQLPlus logged in as SYSDBA
You say SQL database. Most databases including Oracle can be queried using the SQL langauage. If you are talking about Microsoft SQL Sever you are in the wrong place and are mis-using the term SQL which stands for Structured Query Language. A quick search on the internet can get you the answer you are looking for.
The issue has been fixed.
We have restarted the database and then changed the single user mode to multi user mode.
Now we can able to access the database.